Re: Race in pagevec code

[Christian Ehrhardt]

On Wed, Aug 21, 2002 at 10:41:48AM -0700, Andrew Morton wrote:
> Christian Ehrhardt wrote:
> > 
> > ...
> >       Both processors succeeded in bringing the page_count to zero,
> >       i.e. both processors will add the page to their own
> >       pages_to_free_list.
> 
> This is why __pagevec_release() has the refcount check inside the lock.
> If someone else grabbed a ref to the page (also inside the lock) via
> the LRU, __pagevec_release doesn't free it.

I saw this check but this doesn't help. There is no guarantee that this
other reference that someone grabbed is still beeing held at the time
where we do the check:
The problem is if this newly grabbed reference is again dropped BEFORE
the check for page_count == 0 but AFTER put_page_test_zero. In this
case there can be TWO execution paths the BOTH think that they dropped
the last reference, i.e. both call __free_pages_ok for the same page.
See?

> So the rule could be stated as: the page gets freed when there are
> no references to it, presence on the LRU counts as a reference,
> serialisation is via pagemap_lru_lock.

I assume you mean zone->lru_lock here, pagemap_lru_lock is gone
in 2.5.31+everything.gz. Besides this would possibly help if all accesses
to the page_count were protected by the lru_lock (I know we don't want
this). Relying on the lru_lock to protect certain "critical" accesses to
page_count against each other sounds dangerous to me.

> > I don't have a fix but I think the only real solution is to
> > increment the page count if a page is on a lru list. After all
> > this is a reference to the page.
> 
> One would think so, but that doesn't really change anything.

We'd need yet another kernel thread that removes pages with no other
references from the lru_list. I agree that this is probably not an
option. What we really want is to make the lru_cache reference a weak
reference to the page.

    regards   Christian

-- 
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